Electricity is produced, transmitted and distributed at an oscillating frequency of 50 or 60 Hz. Transformers are electrical apparatus used for modifying electricity oscillating voltage and current. Because of the transformer, produced electricity can be transformed and transmitted at high voltage and low current over long distances with minimal joule loss before being transformed again to a lower voltage. Distribution transformers are located at the end of the power lines to reduce the voltage to usable values. Conventional distribution transformers comprise discrete primary and secondary electrical conducting coils each made by winding multiple loops of an electrical conductor while providing proper voltage insulation between loops. Both primary and secondary coils enlace a core made of a ferromagnetic alloy to create a path for a magnetic flux to circulate in a closed loop through both of said coils. When an oscillating voltage is applied across the leads of the primary coil, it induces a fluctuating magnetic flux in the core which, by reverse effect, induces back a voltage across the lead terminals of the secondary coil. The ferromagnetic property of the core allows an increase of the peak magnetic flux density induced by the coils in order to reduce the coils loop size and therefore to reduce the size of the transformer. The primary coil of a distribution transformer is on the high voltage side, and the secondary coil is on the low voltage side. More than one secondary coil may enlace the core to provide more than one low voltage.
For households and small plants which account for the majority of loads, most distribution transformers installed on the grid are single-phase units having a load capacity mostly ranging from 10 to 200 kVA. A typical primary voltage ranges between 5 to 30 kV, and the secondary voltage ranges from 110 volts to up to 480 volts. The coils and the core form an interlaced assembly and are generally attached in an enclosure filled with dielectric oil and which is equipped with feed-through bushings for electrically connecting the leads (an insulated electrical conductor connected to an electrical device) of the coils respectively to the power line and to the load. For purpose of clarity, the term “transformer kernel” in this document will refer to the coils and core assembly without the enclosure and accessories.
Two main types of transformer kernels are used for producing distribution transformers: shell-type and core-type. In the shell-type design, the return flux paths in the core are external to the enclosed coils. This is not the case for the core-type design. A single phase shell-type distribution transformer has two cores respectively enlaced around two distinct limbs of a single enclosed winding arrangement comprising the primary and secondary coils. Alternately, a single phase core-type transformer has two winding arrangements respectively enlaced around two limbs of a single enclosed core. If the distribution transformer has two secondary coils, typically for outputting 120/240 volts, then each secondary coil in the core-type design must be subdivided and distributed among both winding arrangements to ensure proper operation of the transformer under unbalanced loading; otherwise, excessive tank heating will result from magnetic flux leakage. Doing so requires making additional connections to link in series lead terminals from the sub-coils composing each secondary coil. Shell-type transformers may operate correctly with non-subdivided secondary coils. However, subdividing the secondary coils provides a balanced impedance for each 120-volt circuit, thus resulting in better voltage regulation, minimizes circulating current in the secondary coils when connected in parallel to supply one phase of a three-phase distributed voltage, and offers greater coil reliability against lightning surges. Many manufacturers produce non-subdivided secondary coils in their shell-type distribution transformers to avoid the need for additional connections, which requires bulky leads that significantly increase the radial builds of the coils, often resulting in a requirement for a larger tank. As making these connections is done manually, it is believed that the reliability can be increased by avoiding them, which also reduces the cost of the transformer while still providing acceptable voltage regulation and circulating current characteristics.
Choices of conductor materials for making the coils are limited to copper and aluminum. Copper is more conductive than aluminum but weighs more. Insulating materials mostly used are oil, kraft or aramid paper, cardboard, pressboard, varnish, resin epoxy or reinforced epoxy. There are more available choices of magnetic materials for making the core. Two family types of ferromagnetic alloys are mainly used for making distribution transformer cores: grain-oriented silicon-steels and amorphous-steels. Other alloys are available but are not cost effective and are targeted for making motors, high frequency cores.
Grain-oriented silicon-steels are crystalline alloys formed in multiple interrupted steps involving casting; annealing; quenching; rolling; decarburisation; and coating, which come out into sheet thicknesses ranging from 0.23 to 0.35 mm and in different grades. Their crystal grains are oriented in the sheet in order to provide uniaxial magnetic anisotropy which is parallel to the rolling direction. Uniaxial anisotropy reduces the transformer exciting current and core loss. The sheet must therefore be positioned within the transformer core to have the rolled direction following the circulating path of the induced magnetic flux. Prior to being used, silicon-steel sheets must be heated in the form they will occupy in the core in order to relieve applied bending stresses that impair on the magnetic properties and in order to retain the shape. Heating of the steel is generally performed by annealing the preformed cores in batch for a prolonged time in a furnace at a temperature above 800° C.
Amorphous-steels are non-crystalline alloys formed by casting the melted alloy on the surface of a cooling wheel rotating at high speed, which will form a ribbon having a thicknesses measuring from 0.02 to 0.05 mm. They are cheaper to form than silicon-steels because of the single step casting process advantage. Even when considering composition of both alloys, the price of as-cast amorphous steel ribbons is cheaper than most popular grain-oriented silicon-steels. Amorphous-steels also need to be heated to relieve internal residual stresses incurred during casting and due to applied bending stresses. In addition, it is preferable to anneal amorphous-steels in a magnetic field to reduce the coercive force and to induce uniaxial magnetic anisotropy which will be in parallel with the applied field. Conventionally, the amorphous-steel ribbon is positioned within the transformer core to have its longitudinal axis oriented following the circulating path of the induced magnetic flux. Cores are conventionally batch annealed in a furnace at a temperature above 300° C. and with an applied magnetic field that follows the circulating path. Following annealing, the amorphous-steel cores remain very sensitive to externally applied stresses and the ribbon is rendered brittle. This makes amorphous-steels cores difficult to handle and to assemble with the coils.
Internal power loss is inherent to all distribution transformers as they degrade their efficiency and, efficiency of distribution transformers is an important aspect for energy savings considerations. Internal power loss is generated in the transformer when it is energized and it increases during loading. When only energized, an induced fluctuating magnetic flux density is continuously present in the core. This generates core loss in the ferromagnetic material associated with the magnetization cycle and induces current loops within the metal alloy which create joule loss. At a same level of fluctuating magnetic flux density, amorphous-steel cores produce about one third of the core loss generated in silicon-steel cores. When a load is added, load currents flowing in the conductors of the transformer coils produce additional joule loss that is inversely proportional to the conductor size. At a same size, a copper conductor will heat less than an aluminum conductor. The efficiency of the transformer will be the ratio of the outputted power with respect to the total of outputted power and internal power loss (core and coils).
The distribution transformer power output capability is rated based on the temperature rise above ambient of the coils. Immersing the transformer kernel in oil contained in a smooth tank is the most economic means for providing efficient cooling for keeping the coils temperature rise within permissible limits. Heat is transferred from the hot transformer kernel to the oil, from the oil to the tank walls, and then from the tank walls to the outside. For higher cooling capability, the wall surface of the tank can be increased by corrugation or by means of either external metal tubes welded into the sides of a tank or by external radiators attached to the plain tank. Heat removal from the transformer kernel must also take into account the transfer of heat from the interior parts of the kernel. Heat can be transferred by means of conduction, radiation, and convection. Of all three, convection is the most important. Convection occurs by exposing hot surfaces to oil. Heat conducted from the hot surface to the oil increases the fluid temperature and decreases its density. This produces a circulating current as the lighter hot oil moves up in the tank to be replaced by the cooler heavier fluid. The hot oil will be cooled by convecting along the tank surface and will go back to the bottom. An increase of the heat transfer can be gained by creating ducts on or within the kernel, mainly in the coils, with one opening located near the bottom of the tank and another near the top for oil to flow through. This will increase the oil convective current by a chimney effect (or stack effect).
Proper choice of material and sizing of the transformer kernel will have an impact on the transformer efficiency and rating. At equivalent transformer efficiencies, the significant subtracted core loss gained over silicon-steel cores by using amorphous-steel cores can be transformed into additional joule loss within the coils by using smaller conductor sizes. Doing so has the advantage of reducing transformer size but will increase the joule loss density within the coils. This extra heat in the coils may become a problem for evacuation if proper heat transfer means cannot be provided from the interior of the coils to the oil.
Manufacturing of coils generally involves winding conductors and paper sheets on a coil form using semi-automated or fully automated winding machines. The primary and secondary coils have distinct numbers of coiled loops (winding turns). The number of coiled loops of the primary coil will establish the peak magnetic flux in the magnetic path of the core and the ratio of coiled loops of the primary coil over the secondary coil will transform the input voltage to be outputted across the secondary coil. The high voltage primary coil is normally made of several coiled loops arranged in stacked rows (in the present document, a row is a number of objects arranged in a “straight line”) of a small conductor size covered by a varnish to insulate the side by side stacked conductors. Preferably, a paper is added between adjacent rows to provide an increased voltage withstanding capability. A spacer may be added in between adjacent rows to provide cooling ducts for the coil. The low voltage secondary coil having fewer turns, it becomes cheaper to wind a single width of a bare conductor strip side by side with a wider paper strip. Exit leads must be provided on the side of the coils at both ends of the coiled conductor to allow connection between coils or to the feed-through bushings. The lead terminals are normally welded to the ends of the coiled conductor, and are dressed with insulating sleeves to ensure proper voltage isolation. Lead terminal installation, dressing and connection with the bushing are mostly done manually, which increases transformer costs.
Manufacturing of transformer cores involves adjoining or overlapping multiple stacked flat ferromagnetic metal sheets (stacked-cut-core), or butting or overlapping both ends of metal sheets that are bent into a closed loop shape (wound-cut-core), or rolling up multiple turns of a continuous strip of metal sheet (rolled-up-uncut-core). Cut cores have significant disadvantages over uncut cores. Firstly, manufacturing of cut cores involve a lot of labour for cutting and forming, which increases transformer costs. Secondly, an increase in transformer exciting current and power loss are associated with the presence of joints in the cores. Thirdly, cut cores lose the ability to withstand hoop stresses and must therefore must be strapped and framed to prevent the joints from opening. Of all types of cores, amorphous-steel wound-cut-cores are the most expensive to produce, as they require, among other things: more cutting steps; a special annealing furnace, and careful post handling.
Interlacing the electrical coils with the magnetic core is achieved either by manually opening and reclosing the cut core around the pre-wound electrical coils, or by winding the conductor of the electrical coils around a limb of the magnetic core (cut or uncut), or inversely, by rolling up the metal strip around a limb of the coils to form an uncut core. In the second and third cases, the coils to be wound or the uncut core to be rolled up must be of circular shape. However, winding a continuous conductor or metal strip around a limb is a smooth continuous task better adapted for mass production in an automated industrial process. In a transformer having circular coils, it is preferable to have a core limb of substantial cylindrical shape to maximize the filling of the window of the coils in order to minimize the size of the transformer. Therefore, different steel sheet widths must be stacked or wound to create a core cross-section delimited by a circular boundary. Manufacturing of such a core requires production of magnetic strips of different widths or material slitting and more labour. On the other hand, a transformer having a circular core is preferable as each conductor coiled loop can be arranged in the coils for all of them to occupy most of the circular window of the core. Accordingly, the electrical conductors are wound with a different number of coiled loops per row to fit within a circular boundary. This can be done with the small electrical conductor commonly constituting the high voltage primary coil. Generally, the primary coil is made with circular conductors, but may also be made with rectangular conductors for a better filling of the circular window. However, coiling and stacking up different widths of rows of conductors to fit within a circular boundary, and winding an insulating sheet between adjacent rows is not obvious as conductors would tend to pack themselves in a distorted manner and break the row, especially at the ends of each row near the edges of the circular boundary, thereby creating difficulties in winding the insulating sheet without tearing the edges. Therefore, ensuring organisation of coiled conductors in stacked rows is critical as insulating sheets must be wound between adjacent rows. As for the low voltage secondary conductors, they are generally produced by simultaneously coiling a large bare conductor strip and an insulating strip so that they pile up at each coiled loop in the middle portion of the circular area with the rows of the conductors of the high voltage primary coil being distributed on both sides. The assembled coil must then provide gaps between the edges of the window of the coils to allow rotation of the core to be formed. Core lacing can be made simple by using coils having an overall rectangular shape in order to provide a rectangular window in the coils for rolling up a single width continuous ferromagnetic metal strip. If necessary, a two-part rotating mandrel can be installed on the limb of the coils as a support to easily roll up the strip.
A circular magnetic uncut core can be manufactured using a continuous single strip width of a silicon-steel sheet or, of an amorphous-steel ribbon. For silicon-steel, the complete core must first be rolled up on a second mandrel having the same diameter as the one mounted on the coil limb and then annealed in a furnace. Once annealed, the core must be unrolled and rolled up again on the electrical coils by inserting the internal end of the strip first in order to put the strip back to its annealed configuration. Given that conventional grain-oriented silicon-steel sheets show significant stiffness, proper care must be taken to roll up the strip without bending the material beyond its elastic limit. This makes the rolling up process more difficult and the cores still need to be rolled up at first, annealed and handled separately, which increase transformer costs. Annealed amorphous-steel circular uncut cores, on the other hand, are built with a ribbon so thin that it can be severely bent during transfer without reaching plastic deformation. Because the alloy remains sensitive to externally applied stresses once annealed, tightly rolling up the ribbon increases core loss and exciting currents. The best practice is to gently roll up the ribbon at a low tensile stress and to hold the finished core in place without adding significant stresses with the framing structure. However, when the ribbon is rolled up at low tensile stress, the formed core has no self structural integrity and the ribbon can easily telescope if allowed to slide at one end. If the core axis is positioned vertically, a supporting base is required. Also, because amorphous steel gets very brittle following the furnace annealing treatment, unrolling and rolling up again is not obvious. The method of unrolling, transferring and rolling up on another mandrel of a ribbon from a furnace annealed ferromagnetic amorphous steel circular core has been considered in U.S. Pat. No. 4,668,309 and in many articles such as: “Induction Accelerator Development for Heavy Ion Fusion”, L. L. Reginato, IEEE Proceedings of the 1993 Particle Accelerator Conference, vol. 1, p. 656-660, and: “Exciting New Coating For Amorphous Glass Pulse Cores”, R. R. Wood, IEEE 1999 12th International Pulsed Power Conference, vol. 1, p. 393-396, and: “Induction Core Alloys for Heavy-ion Inertial Fusion-energy Accelerators”, A. W. Molvik, The American Physical Society, Physical Review Special Topics—Accelerators and Beams, vol. 5, 080401, 2002. From these prior art analyses, this method is believed to be impractical as the ribbon tends to break too often during the transfer because of its severe brittleness.
Another important aspect of a distribution transformer kernel is its ability to withstand a short-circuit fault at the output of the secondary coil. During short-circuit conditions, repulsive forces are generated between the primary and secondary coils. These repulsive forces act on the coils in a way that they want to adopt a circular shape. These forces will not impair on the structural integrity of the coils if they are already made in a circular shape or, they can be sufficiently alleviated using elliptical shape coils but this would require winding a series of strips having different widths which will increase transformer cost as stated above. A core formed by rolling up a single width steel strip will have a rectangular cross-section. Therefore, the coils must have 4 straight limbs delimiting a rectangular window for the core rectangular cross-section to pass through. Conductors and paper forming rectangular coils lack the self structural integrity required to withstand the repulsive forces. Strong inward forces will appear at each corner of the coils, which may cause the insulation at the corners to fail if no adequate support is provided at the corners. Support at the corner may be provided if the coils are firmly leaning against the core. A silicon-steel circular core may be strong enough to sustain the inward forces on its corners, but this is not possible for an amorphous steel circular core having poor structural integrity and which reacts negatively to applied stresses as stated above. Coiling a bandage around the coils or impregnating the coils in resin will improve the mechanical strength to a certain extent. Proper self mechanical strength can be obtained by encapsulating the coils in a casting resin to provide external reinforcing structure. However, care must be taken to ensure that no bubbles are trapped during the casting to avoid a corona discharge. There is also a risk of de-lamination between the conductors/paper and the resin. Furthermore, cooling of the coils is made more difficult.
Once a transformer kernel is completely assembled, means must be provided to attach and secure the kernel in the enclosure. Conventional silicon-steel distribution transformers are solidly fixed to the enclosure via the core. The practice is to use the core as a support for the coils and then to clamp and secure the core in the enclosure with a frame. This method is not suitable for amorphous-steel distribution transformers and attaching the kernel via the coils is impractical. A best practice for amorphous-steel distribution transformers is to have a frame for supporting both the coils and the core without stressing them too much.
The U.S. Pat. No. 5,387,894 discloses a core-type distribution transformer comprising a circular core made by rolling up a continuous strip of ferromagnetic material on a mandrel located around the cylindrical shape of two adjacent windings having a limb of semi-circular cross section. The ferromagnetic strip can be an amorphous steel strip which was first rolled up on a mandrel and then annealed under magnetic saturation before being unrolled and rolled up again around the coils. However, the document does not discuss the embrittlement of the amorphous steel strip occurring after core annealing and the difficulty to transfer a brittle amorphous-steel strip and, does not teach how to provide support to the formed amorphous steel core. Additionally, no teaching is provided on how to efficiently coil stacked up rows of conductors of different widths and to wind an insulating sheet between adjacent rows of conductors within a circular boundary. Additionally, no teaching is provided on how to make, locate and connect the conductor lead terminals exiting from the coils. Additionally, no means are provided to secure the adjacent windings and to secure the transformer kernel into the tank. Additionally, no mechanical structural means are provided to the coils to adequately sustain short-circuit mechanical forces at the corners of the rectangular coils as the document alleviates the problem by making these corners of the coils curved in an elliptical configuration. Finally, the document does not teach how to provide cooling means to transfer the heat generated in the conductors outside of the coils, especially when using an amorphous steel core where such heat is more intense and is being generated in smaller coils for a given transformer efficiency.